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Ganymede
Ganymede is the largest moon in the Solar System. With a diameter of 5268.2 km it is 2000 km larger than the Moon. It is the third Galilean sattelite, and the 7th innermost moon of Jupiter. Ganymede is in a 1:2:4 resonance with Io and Europa. It is larger than the planet Mercury. On January 11, 1610, Galileo Galilei observed what he believed were three stars near Jupiter; the next night he noticed that they had moved. He found a fourth supposed star, which would turn out to be Ganymede, on January 13. By January 15, Galileo came to the conclusion that the stars were actually bodies orbiting Jupiter. Orbital mechanics Ganymede orbits Jupiter at a distance of 1 070 400 km, third among the Galilean satellites, and completes a revolution every seven days and three hours. Like most known moons, Ganymede is tidally locked, with one face always pointing toward the planet. Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing quasi-periodically due to solar and planetary gravitational perturbations on a timescale of centuries. The ranges of change are 0.0009–0.0022 and 0.05–0.32°, respectively. These orbital variations cause the axial tilt (the angle between rotational and orbital axes) to vary between 0 and 0.33°. Ganymede participates in orbital resonances with Europa and Io: for every orbit of Ganymede, Europa orbits twice and Io orbits four times. The superior conjunction between Io and Europa always occurs when Io is at periapsis and Europa at apoapsis. The superior conjunction between Europa and Ganymede occurs when Europa is at periapsis. The longitudes of the Io–Europa and Europa–Ganymede conjunctions change with the same rate, making the triple conjunctions possible. Such a complicated resonance is called the Laplace resonance. Composition The average density of Ganymede, 1.936 g/cm3, suggests a composition of approximately equal parts rocky material and water, which is mainly in the form of ice. The mass fraction of ices is between 46–50%, slightly lower than that in Callisto. Some additional volatile ices such as ammonia may also be present. The exact composition of Ganymede's rock is not known, but is probably close to the composition of L/LL type ordinary chondrites, which are characterized by less total iron, less metallic iron and more iron oxide than H chondrites. The weight ratio of iron to silicon is 1.05–1.27 in Ganymede, whereas the solar ratio is around 1.8. Ganymede's surface has an albedo of about 43%. Water ice seems to be ubiquitous on the surface, with a mass fraction of 50–90%,2 significantly more than in Ganymede as a whole. Near-infrared spectroscopy has revealed the presence of strong water ice absorption bands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 μm. The grooved terrain is brighter and has more icy composition than the dark terrain. The analysis of high-resolution, near-infrared and UV spectra obtained by the Galileo spacecraft and from the ground has revealed various non-water materials: carbon dioxide, sulfur dioxide and, possibly, cyanogen, hydrogen sulfate and various organic compounds. Galileo results have also shown magnesium sulfate (MgSO4) and, possibly, sodium sulfate (Na2SO4) on Ganymede's surface. These salts may originate from the subsurface ocean. Magnetic field Ganymede is the only satellite in the Solar System known to possess a magnetosphere, likely created through convection within the liquid iron core. The meager magnetosphere is buried within Jupiter's much larger magnetic field and connected to it through open field lines. The satellite has a thin oxygen atmosphere that includes O, O2, and possibly O3 (ozone). Atomic hydrogen is a minor atmospheric constituent. Whether the satellite has an ionosphere to correspond to its atmosphere is unresolved. Atmosphere In 1972, a team of Indian, British and American astronomers working at Indonesia's Bosscha Observatory claimed that they had detected a thin atmosphere around the satellite during an occultation, when it and Jupiter passed in front of a star. They estimated that the surface pressure was around 1 μBar (0.1 Pa). However, in 1979 Voyager 1 observed an occultation of a star (κ Centauri) during its flyby of the planet, with differing results. The occultation measurements were conducted in the far-ultraviolet spectrum with wavelength shorter than 200 nm; they were much more sensitive to the presence of gases than measurements in the visible spectrum in 1972. No atmosphere was revealed by the Voyager data. The upper limit on the surface particle number density was found to be 1.5 × 109 cm−3, which corresponds to a surface pressure of less than 2.5 × 10−5 μBar.49 The latter value is almost five orders of magnitude less than that measured in 1972, indicating that the earlier interpretation was too optimistic. Despite the Voyager data, evidence for a tenuous oxygen atmosphere on Ganymede, very similar to the one found on Europa, was found by the Hubble Space Telescope (HST) in 1995. HST actually observed airglow of atomic oxygen in the far-ultraviolet at the wavelengths 130.4 nm and 135.6 nm. Such an airglow is excited when molecular oxygen is dissociated by electron impacts, evidence of a significant neutral atmosphere composed predominantly of O2 molecules. The surface number density probably lies in the 1.2–7 × 108 cm−3 range, corresponding to the surface pressure of 0.2–1.2 × 10−5 μBar. These values are in agreement with the Voyager's upper limit set in 1981. The oxygen is not evidence of life; it is thought to be produced when water ice on Ganymede's surface is split into hydrogen and oxygen by radiation, with the hydrogen then being more rapidly lost due to its low atomic mass.50 The airglow observed over Ganymede is not spatially homogeneous like that over Europa. HST observed two bright spots located in the northern and southern hemispheres, near ± 50° latitude, which is exactly the boundary between the open and closed field lines of the ganymedian magnetosphere (see below). The bright spots are probably polar auroras, caused by plasma precipitation along the open field lines. Ionosphere The existence of a neutral atmosphere implies that an ionosphere should exist, because oxygen molecules are ionized by the impacts of the energetic electrons coming from the magnetosphere and by solar EUV radiation. However, the nature of the Ganymedian ionosphere is as controversial as the nature of the atmosphere. Some Galileo measurements found an elevated electron density near the moon, suggesting an ionosphere, while others failed to detect anything. The electron density near the surface is estimated by different sources to lie in the range 400–2,500 cm−3. As of 2008, the parameters of the ionosphere of Ganymede are not well constrained. Additional evidence of the oxygen atmosphere comes from spectral detection of gases trapped in the ice at the surface of Ganymede. The detection of ozone (O3) bands was announced in 1996. In 1997 spectroscopic analysis revealed the dimer (or diatomic) absorption features of the molecular oxygen. Such an absorption can arise only if the oxygen is in a dense phase. The best candidate is the molecular oxygen trapped in ice. The depth of the dimer absorption bands depends on latitude and longitude, rather than on surface albedo—they tend to decrease with increasing latitude on Ganymede, while the O3 shows an opposite effect. Laboratory work has found that O2 would not cluster and bubble but dissolve in ice at Ganymede's relatively warm surface temperature of 100 K. Origin Ganymede likely formed by an accretion in Jupiter’s subnebula, a disk of gas and dust surrounding Jupiter after its formation. The accretion of Ganymede probably took about 10 000 years, much shorter than the 100 000 years estimated for Callisto. The Jovian subnebula may have been relatively "gas-starved" when the Galilean satellites formed; this would have allowed for the lengthy accretion times required for Callisto.64 In contrast Ganymede formed closer to Jupiter, where the subnebula was denser, which explains its shorter formation timescale. Category:Moons Category:Jupiter Category:Moons of Jupiter